Electrolytic process and apparatus



Jan. 9, 1951 c. coN rr 2 ,537,304

ELECTROLYTIC PROCESS AND APPARATUS Filed Oct. 7, 1946 3 Sheets-Sheet 2 o'0 g g f WWW/ o ilj :g L L T] m r 2 lnveniof PA LC. COND/T AiiorneysJan. 9, 1951 P. c2. CONDIT 2,537,304

ELECTROLYTIC PROCESS AND APPARATUS Filed Oct. 7, 1946 5 Sheets-Sheet 5 g8 Inventor Attorneys Patented Jan. 9, 1951 I ELECTROLYTIC PROCESS ANDAPPARATUS Paul C. Condit, Berkeley, Calif., assignor to CaliforniaResearch Corporation, San Francisco, Calif., a corporation of DelawareApplication October 7, 1946, Serial No. 701,761

4 Claims.

This invention relates to a process and apparatus for electrolytictreatment of chemical compositions and, more particularly, to a processand apparatus for effecting electrolytic oxidation and reductionreactions.

An object of the invention is to provide an improved electrolyticprocess and apparatus for continuously treating chemical compositions toproduce oxidation-reduction reactions with relatively high yields ofreaction products while simultaneously maintaining both high currentefficiency and superior cell capacity;

Additionally an object of this invention is the provision of anelectrolytic cell for treating or producing sensitive chemicals whileminimizing undesirable alteration thereof b reducing the contact timerequired for conversion in the cell.

Another object of the invention is to provide a non-cloggingelectrolytic cell capable of handling a continuously flowing, thinribbon or film of an electrolyte carrying the chemical composition to betreated.

It is also an object of the invention to provide a process and apparatuscapable of continuous and prolonged operation with substantially no cellpoisoning. I

A further object of the invention comprises the provision of acontinuous process for cathodically reducing a chemical composition inhigh yield and with high current efficiency.

Another object is to furnish an electrolytic reduction process andapparatus capable of effecting reduction in low treating time wherebysensitive chemicals may be treated or produced with maximum desiredconversions and minimum deleterious alteration.

Additionally, an object is to furnish a cathodic reduction processwherein side reactions are minimized, currentv eificiency is increased,and the cell capacity is enhanced by passing an electric currenttransversely to the direction of fiow of a hydrodynamically balancedfilm or ribbon of an electrolyte carrying an electrolytically reduciblechemical composition.

A further object is to provide a process of electrolytically treating achemical composition in thin fluid films or ribbons while avoidingpossibilitiesof clogging or impeding proper flow of said films by reasonof inadvertent introduction of foreign particles or bodies.

Another object is to provide a cathodic reduction process in which cellpoisoning substantially is eliminated.

Other objects and advantages of the invention will be apparent from thefollowing description and drawings, in which Figure l is asemi-diagrammatic illustration of a hydraulic system in ,an electrolyticcell and of a process embodying the principles of this invention. Figure2 is an enlarged and somewhat detailed cross-section of a presentlypreferred embodiment of the inven tion. Figure 3 is a diagrammaticfiowsheet of a plant or system for effecting electrolytic reactions inaccordance with this invention.

Figure 4 is a transverse sectional view of a modified form of apparatuswhich may be employed in the practice of the invention- Figure 5 is afragmentary longitudinal sectional view of a portion of the apparatusillustrated in Figure 4.

Figure 6 is a longitudinal sectional view of a further embodiment of theinvention.

Figure 7 is a fragmentary sectional view of a portion of the apparatusshown in Figure 6.

Figure 8 is a longitudinal sectional view of a third form of apparatuswhich may be employed in the practice of the invention.

The process of this invention utilizes a flowing stream of anelectrolyte in hydrodynamic balance between a semi-"permeable diaphragmand a cell electrode, said diaphragm and electrode being in hydraulicbalance.

In accordance with a preferred embodimentof therinvention a chemicalcomposition to be treated is dispersed in a liquid electrolyte to formtially to follow the contour of the cell diaphragm surface b selecting adiaphragm having a 'surface which is preferentiall wetby the liquid dispersion. Desirably, .the diaphragm surface should not be wet by thesecond immiscible liquid in order that the liquid dispersion will spreadmore readily into the desired film or ribbon, more faithfully follow thecontour of the diaphragm surface, and give greater cell efficiency asshown hereinafter; V V

A, clearer understanding of theqforeg oing may be had by reference toFigure 1 of the drawing, wherein it represents a cell diaphragm of thesemi-permeable type; i. e., V a diaphragm relatively highly permeable toions of the electrolytic solution when under electrical potentialgradient and relatively impermeable to cell liquids under simplehydraulic pressure. A thin stream of electrolyte I l flows along andfollows the contour of the diaphragm surface as illustrated and is inhydrodynamic equilibrium with the hydrostatic pressure of the liquidmercury electrode l2. It should be observed that, as here shown, theelectrolyte stream is in the form of a thin film squeezed between thesemipermeable diaphragm I ll, on the one hand, and the liquid mercuryelectrode IE, on the other hand. The hydrostatic liquid pressure of themercury electrode successively increases with the depth of the mercuryand this pressure serves, first, to force the electrolyte stream into athin film and, secondly, to provide a hydraulic slope for saidelectrolyte stream causing the same to flow upwardly along the surfaceof the porous diaphragm as indicated. Desirably, the mercury electrodedoes not wet the surface of the diaphragm, whereas, as here illus--trated, the electrolyte perferentially wets said surface and therebyfaithfully follows the contour of said surface rather than break awayinto the liquid mercury electrode. It should be apparent that the liquidelectrolyte phase and the liquid electrode phase should be mutuallyimmiscible to avoid dilution or dispersion each by the other. In thissystem the electrolyte stream flows at such a rate and in such athickness as will place it in hydrodynamic equilibrium with theessentially static liquid pressures exerted by the liquid mercuryelectrode transversely to the direction of flow. Cell electric currentlikewise is passed through said electrolyte stream transversely to thedirection of stream flow for effecting the desired chemical change incompositions dispersed or contained in said electrolyte liquid.

The foregoing system has various important advantages in both itselectrolytic and mechanical aspects. Mechanically, the cell isnonclogging and automatic in operation features not heretofore obtained,so far as applicant is aware, in any truly continuous cell of thesemi-permeable diaphragm type. When and if a foreign particle whichnormally would clog or plug a narrow passage comparable in thickness tothat of the electrolyte stream is introduced into the present cell, theparticle does not become mechanically wedged since the fluid pressure ofthe liquid mercury gives 'way to said particle and allows it to flowalong with the electrolyte stream. Even if, by some peculiarcircumstance, said particle should become fixed in the line ofelectrolyte flow, it is likewise apparent that the electrolyte may passaround said particle since the liquid mercury electrode is inhydrostatic balance with the flowing stream. Further, the hydrostaticpressure of the liquid mercury and the hydrodynamic forces of theflowing e ectrolyte stream automatically regulate the thickness of thefilm for any given flow rate since these forces are in equilibrium.Mechanical adjustments or controls for spacing cell electrodes toregulate film thickness are avoidedregulation of electrolyte flow ratebeing sufficient.

An additional advantage of this hydrodynamically balanced system is thatcurrent density appears to be automatically adjusted and correlated withconcentration of reactant to give enhanced current efiiciency and highyields in the electrolytic reduction of organic compounds. The exactexplanation for this automatic correlation and for the outstandingefiiciency of this system has not been established, but it is believedthat since (1) rate of flow of the electrolyte stream through the cellis maintained substantially constant and (2) the total electric currentflowing through the cell may likewise be maintained substantiallyconstant, it is the current density variable which is automaticallyregulated by this invention to yield improved performance. (It is knownthat proper regulation of current density is a most important variablefor controlling cell efliciency where other factors are held constant asabove stated.)

Such automatic regulation of current density can be understood when itis considered that in the electrolytic reduction of organic compounds,the compound being reduced acts as a depolarizer. The electrodes used incathodic reduction are subject to polarization and tend to develop arelatively high hydrogen over voltage. Polarization at the surface ofthe electrode increases the resistance to flow of electric currenttherethrough, and conversely the action of depolarizers serves todecrease the electrical resistance to the current and hence serves toincrease current density through the cell at the point wheredepolarization occurs most effectively.

Keeping in mind the foregoing principles, it will be seen that theconcentration of the organic compound being reduced (depolarizer) isgreatest in the zone where the electrolyte enters the cell and reductionbegins. This concentration is continuously diminished as the electrolytestream progresses through the cell by reason of the electrolytic actionof said cell in chemically converting the organic compound to itsreduced form, in which form it no longer acts as a depolarizer. Theresulting lower concentration of depolarizer in turn is thought todecrease depolarization and conversely to allow increased polarizationat the surface of the mercury electrode as the electrolyte streamprogresses through the cell. In accordance with the foregoing theory ahighly depolarized, low resistance mercury electrode surfaceautomatically is produced where the electrolyte first enters the cell,and a more polarized, higher resistance mercury electrode surface isobtained when and where electrolytic reduction is substantiallycompleted. Thus, polarization and electrical resistance thusprogressively increase from the point of entry along the electrolytestream to the point of exit, and current density through the streamconversely is progressively decreased from the point of entry to thepoint of exit or complete reduction. This automatically maintains arelatively high current density at the point of entry and likewiseautomatically diminishes said current density progressively through thecell in direct correlation with the concentration of unreduced organiccompound being treated. This correlation of current density withconcentration of unreduced chemical in the flowing electrolyte stream isbelieved to be responsible for the remarkably high efficiency of thepresent process and apparatus in effecting cathodic reduction reactions.

It should be noted that one of the preeminent problems which has beenencountered in electrolytically inducing reactions with organiccompounds has been the accumulation or formation of cell poisons duringcell operation. Little is known concerning the nature of these cellpoisons nor is the mechanism of their poisoning action adequatelyunderstood, but their effect is unequivocally deleterious and theunsolved cell poisoning problem in many instances has been largelyresponsible for the limited acceptance of proposed electrolyticsynthesis of organic com- In the above equations, F) represents afaraday of electricity and the remaining symbols have their usualchemical significance. Cell poisons increase the extent of reaction (2)above or other side reactions and hence decrease either the desiredconversion by reaction (1) 01' the efiiciency of the cell; or both. Thepractical feasibility of the entire electrolytic process de-' pends uponthe extent to which reaction (1) can be effected to the exclusion ofreaction (2) above, and, in turn, therefore, on the extent to which cellpoisoning can be ayoidedother factors being constant. The utilityof thepresent invention will be readily understood when it is appreciated thatinaddition to the various advantages or simplicity and initialcellemciency the process and apparatus disclosed hereinafter are capableof substantially eliminating cell poisoning in a cathodic reduction.

The invention in its broader aspects, particularly the utility of theapparatus, is not limited to cathodic reduction but includes theelectrolytic treatment of other compounds for other purposes, such asanodic oxidation. However, in order to simplify the description, theinvention is here illustrated by an apparatus and process particularlyadapted to cathodic reduction and more especially to the electrolyticselective reduction of only one carbon-to-carbon double bond in thebenzene. ring of a phthalic acid to produce a cyclohexadienedicarboxylic acid. The production of cyclohexadiene dicarboxylic acidsspecifically is exemplified by production of two alternative compoundsin accordance with the following chemical reactions:

boxyhc acid-1,4

Additionally, other electrolytic reductions of organic compounds may beeifected with advantage according to the present invention. Such otherreductions have been carried out by the process and in the apparatus ofthis invention and are exemplified by: reduction of the aldehyde groupin glucose to give hexitols; reduction of the double bond in maleic acidto give succinic acid; reduction of the nucleus in pyridine to givepiperidine and polypiperidyls; reduction of the cyclic amide group incafieine to give desoxycaffeine; reduction of the nitro group. inp-nitroaniline to give p-phenylene diamine; reduction of the carbonylgroup in acetone to give among other products mercury alkyls.

A preferred form of electrolytic cell embodying the principles of thisinvention and particularly adapted for effecting cathodic reductions oforganic compounds is illustrated in Figure 2 of the drawing. Inspectionof this figure will reveal that within a supporting frame having anupper cover plate I 3 and a lower supporting plate l4 secured to eachother by through bolts IE is a cell container ll of glass or any othersuitable material and desirably cylindrical in shape. Container I1 isclamped between the upper cover plate and the lower supportingframe l4,and in order to prevent relative motion the container is fitted into agasket groove or slot E8 in the cover plate.

Container l1 forms the cathode compartment for the liquid cathode and ishere shown filled with liquid mercury l9. Immersed in the liquid cathodeI9 is a semi-permeable diaphragm 2| which in turn serves to form aninner anode com-' partment here illustrated as filled with any suitableanolyte 22. A liquid cooled anode is provided by means of a coiled tube23, of lead or other suitable metal. This anode and anolyte are cooledby conducting water or similar cooling fluid through tube 23. The anode23 is suitable connected to a source of direct electric current asillustrated. The electrical connection with the mercury cathodedesirably is madethrough an upwardly projecting conduit 26 which is infiuid communication with the mercury in the cathode chamber.

In the preferred operation of the cell the liquid mercury cathode iscirculated through the oathode chamber by way of inlet line 26 andoverflow line 21. This continuous replenishment with fresh mercury inthe cathode followed by scrubbing of the discharged mercury ashereinafter described serves to eliminate any cell poisons which mightaccumulate in a particular operation after prolonged operation. However,in the electrolytic reduction of phthalic acid to A3,5-cyclohexadienetransdicarboxylic acid-1,2 poisoning has not been observed in the cellof Figure 2 and mercury circulation may be omitted in certain instancesor merely utilized as a precaution against contamination fromextraneously introduced contaminants.

Semi-permeable diaphragm 2| desirably is of a type which is not wet bythe liquid cathode and preferably has a catholyte contact surface whichis preferentially wet by the catholyte. Preferential wetting of thediaphragm surface by the catholyte and non-wetting by the liquid cathodeyields better cell performance but is not presently regarded asabsolutely essential to operativeness. It has been found that where thecatholyte is aqueous and the liquid cathode desirably is mercury (as inthe cathodic reduction hereinafter specifically described), a suitablesemi-permeable diaphragm is of unglazed porous porcelain.

The diaphragm of Figure 2 is a porous porcelain cup having its upper rimground plane to insure firm seating in slot or gasket 28. This cup whenplaced inside the cathode compartment is held firmly against cover plateIS in slot 28 by the buoyance of the liquid mercury cathode. It has beenfound that the hydrostatic pressure of the mercury tends to force acertain quantity of the electrolyte through the pores of the cup intothe anode compartment. This tendency can be largely or entirely overcomeby a suitable treatment of the porous cup prior to use.

It has been found for example that the porous porcelain diaphragm can besealed by soaking first in hot dilute water glass and subsequently inhot dilute sulfuric acid to precipitate silica in the form of a gel orthe like within the pores of the cup and form a diaphragm which isrelatively highly permeable to ions and relatively impermeable to cellfluids. The porous cup is conditioned after the foregoing pretreatmentby running in a conventional cell for about one-half hour with a currentof amperes, for example. To further indicate the characteristics of such2. treated porous diaphragm it is noted that the electrical resistanceof the diaphragm after treatment in the foregoing manner rose onlyslightly from 0.075 to 0.980 ohm. Relative permeability is illustratedby the fact that when an untreated porous cup was placed in dilutesulfuric acid and a vacuum equivalent to inches of mercury drawn on it,the diaphragm passed 110 cc. through its pores in a half hour. Thetreated cup passed only 3 cc. in the same period and after two hoursonly 4 cc.

Catholyte is introduced into the cell of Figure 2 at a substantiallyconstant rate through inlet tube 29 and is directed against a surface ofporous diaphragm 2 I. As here shown the stream of catholyte 30preferentially wets the diaphragm surface and flows therealong whilebeing subjected to the hydrostatic pressure of the mercury cathode. Thishydrostatic pressure furnishes th hydraulic slope necessary to causeflow of the catholyte along the diaphragm toward the surface of thecathode. The hydrostatic pressure of the mercury cathode simultaneouslyforces the catholyte into thin films or ribbons which followsubstantially the contour of the diaphragm surface. Thus, for any givenfeed rate the velocity of the catholyte as well as the thickness of thecatholyte films or ribbons is governed primarily by the hydrostaticmercury pressure and is such that the catholyte hydraulic system is inhydrodynamic balance with the hydrostatic pressure of the liquid mercurycathode.

In operation, direct electric current flowing through the cell is passedthrough the catholyte films or ribbons as the catholyte progresses alongthe diaphragm surface. The direction of this electric current istransverse to the flowing films or ribbons of catholyt and as here shownis substantially perpendicular to their general direction of flow.

It should be observed that as any given volume of catholyte is followedthrough the cell its electrolytic reduction occurs progressively and newcathode surface likewise is progressively encountered with a minimum ofmixing and with only.

a relatively short contact time in the zone of electrolytic action. Itis important that the total volume of catholyte subjected to treatmentper unit of active cathode surface is extremely small and that contacttime likewise is relatively short as compared with ordinary batch orsemi-continuous operation. This is of particular advantage wheresensitive compounds are treated or formed in the cell, since undesiredalteration of such compounds may be largely avoided. For example in thereduction of orthophthalic acid to A 3,5-cyclohexadiene dicarboxylicacid-1,2, it has been found that the latter compound is sensitive atcell operating temperatures and tends to isomerize to the correspondingA 2,6 acid. This.

isomerization is minimized while simultaneously obtaining excellentconversions and high current efficiency by reason of the low contacttimes attainable with this invention. It should be recalled at thispoint that the electrolytic current density is automatically correlatedwith the concentration of unreduced chemical as the catholyte progressesalong its path through the zone of electrolytic action. In accordancewith theory and the previous explanations given herein, the highestcurrent density occurs in the zone of entry of the catholyte and thelowest current density is found just below the surface of the mercuryelectrode in the zone of exit where electrolytic action ceases.

It has been found that a plain diaphragm surface tends to permitchanneling of the catholyte by allowing flow of a catholyte stream upone side of the cup to the exclusion of other areas. Such channelingdecreases the conversion capacity of the cell and suitable baffle meanspreferably should be provided on or in the diaphragm surfacev to preventsuch channeling action. As shown in Figure 2, the outer surface of theporous diaphragm along which the catholyte flows is provided with ahelical thread having a square profile to furnish such a baffle means.It will be apparent to those skilled in the art that various otherbaffle means may be substituted for the foregoing helical threadingarrangement.

After reaching the surface of the mercury cathode, the catholyte forms aliquid layer 32 and is discharged outwardly by way of overflow conduit2?. The treated catholyte may be collected and reduced product recoveredtherefrom by the method and in the system illustrated in Figure 3.

Performance data exemplifying the utility of the process and apparatusof this invention are given for electrolytic reduction of orthcphthalicacid to A3,5-cyclohexadiene transdicarboxylic acid, 1,2.

OPERATING PROCEDURE At the start of a run. the entire electrolytic cellassembly is placed in a temperature control bath to maintain the celloperating temperature at about 185 F. A sufiicient quantity ofelectrolyte for the run is prepared by dissolving phthalic anhydride in5% sulfuric acid. Unless otherwise indicated the-concentration used is40 grams of anhydride per liter of acid. The electrolyte is heated toabout F. and is maintained at this temperature. The cathode compartmentof the cell is filled with mercury to the overflow line and the anodecompartment is filled with 5% sulfuric acid solution.

The cathode contact is inserted in the side line 26 of the cell, coolingwater started through the anode and mercury circulation begun. Thecatholyte feed is then started, the electric circuit closed, and theamperage adjusted to the desired value. When uniform operation isestablished,

product is collected and recording of data commenced. After operation iscompleted lines may be flushed by pumping through distilled water.

CURRENT EFFICIENCY In the electrolytic reduction of phthalic acid thetheoretical current requirement is 2 faradays per mol or 0.362 amperehr. per gram of phthalic anhydride reduced. For a solution of 40 gramsof phthalic anhydride per liter, this is equivalent to HA9 ampere hoursper liter of electrolyte re- 9 duced. The current efficiency maytherefore be calculated by the following formula:

N -current efiiciency in percentage S=feed rate in liters/hr.P=percentage of feed reduced I=applied current in amperes Currenteificiency represents the per cent of the total applied current consumedin the desired reaction; and the difference between 100 and the per centelficiency is the percentage going to side reactions.

In general it had been found that batch operations utilizing 20 amperessubstantially reduced 750 cc. of electrolyte per hour. In the followingseries of runs data for which are given in Table I, the initial run wasmade under these conditions. However, it was discovered that the cellwas much more efficient than batch cells previously run, and current wasaccordingly reduced in subsequent runs as indicated:

The per cent conversion given in the table is the average of severalanalyses determined on the product crystallized from the catholyteemerging from the cell. A very small amount of unreduced material couldtherefore remain in the mother liquor and not be shown in the analysis.

As may be seen, the current efficiency in a continuous cell of the typeshown in Fig. 2 was much superior to that obtained in batch operation.In later runs at higher feed rates, better efiiciencies were obtainedwith complete conversion as will be apparent from the data exploringcell production capacity.

In batch operations utilizing an electrolytic cell substantiallyidentical in size with the continuous cell illustrated in Fig. 2 andutilized in the runs herein described, reduction of about 750 cc. ofcatholyte per hour was the maximum capacity obtainable with reasonableefficiency. The foregoing series of runs showed that by reason of theenhanced efficiency of the continuous process and apparatus of thisinvention much greater current efficiency could be obtained bymaintaining feed rate corresponding to the production capacity of abatch cell and lowering the current density. A second series of runsrevealed that by maintaining the more efficient or a similar ratio ofcurrent density to feed but vastly increasing the catholyte feed rate,high efiiciency could be obtained while simultaneously increasing cellcapacity many fold. In fact it was found that in general the catholytewould tend to break away from the cup or stream up its sides before thecell capacity was otherwise exceeded. In other words, cell capacity andefficiency are limited primarily by hydraulic capacity and not beelectro-chemical phenomena as had been previously experienced. Thefollowing results were obtained with a cell utilizing a pulsatingcatholyte feed pump which tended to cause the catholyte to break awayfrom the diaphragm surface at a flow rate of about 4 liters per hour.

Table II Feed Current R Applied Per Cent p No 37 23," Amps. ConversionEFSES The system for feeding catholyte to the cell was improved toprovide smooth catholyte flow whereby the catholyte continued to followthe contour of the diaphragm surface at feed rates exceeding 4 litersper hour. sults were obtained after these changes:

Table III Feed Current Applied Per Cent Run Amps. Conversion 12 5 825Some escapeof electrolyte from the diaphragm surface of the cell wasnoticed at 6 liters per hour and this increased slightly as the flowrate was raised. Variations in this escape rate undoubtedly areresponsible for minor discrepancies in the foregoing resultsas is thecase for data given in Table II. Aswill be observed from the data ofTable I'll, even with the minor escape of catholyte from the celldiaphragm, cell efficiency and conversion surpassed that previouslyobtained.

CATHODE POISONING No evidence of cathode poisoning has been observed inthe operation of the electrolytic cells hereindisclosed. In a test runcatholyte was recycled and no poisoning was noticed. In previousoperations with batch and semi-continuous type cells cathode poisoningbecame evident the first time electrolyte was returned to the cell.

CONTACT TIME Comparative contact times were determined for a diaphragmof the threaded cup type shown in Figure 2 and a porous cup of the sameoverall dimension but having a plain non-threaded surface. Averagecont-act time was measured by filling the cathode chamber of theassembled cell to the point of overflow with mercury, starting thecatholyte feed pump, and measuring the quantity of mercury displaced bythe oatholyte in the cell. The measured quantities so obtained are givenbelow for the two types of diaphragm along with contact times calculatedwith these values at the feed rates involved:

Catholyte, cc.

Contact Reference is now made to Figure 3 of the accompanying drawingwhich shows in diagrammatic form a system adapted for commercial op- Thefollowing reeration and utilizing the principles of this invention. Theapparatus comprises an electrolytic cell 1?, similar to that of Figure2, for inducing the desired organic reaction and an inert gas agitatedmixing heater 33 for preparing an electrolyte solution of chemicals tobe treated. A feed pump 3 3 conveys the solution to cell I"! by way ofcell inlet pipe 35. As here shown, the electrolyte solution is acatholyte and after-reduction of the solute as it flows along thediaphragm in the cathode compartment of the electrolytic cell, thecatholyte passes from the cell through discharge conduit 2'! and levelcontroller 36 to any suitable means for effecting product separation. Inthe form here shown, a chiller 3'? is provided for crystallizing theconversion prodnot from the electrolyte solution and a filter 38recovers the crystals from the electrolyte.

The electrolyte used in the cell will depend upon the particularelectrolytic treatment to be effected and upon the chemical compoundselected for the reaction. Many suitable electrolytes, usually aqueous,are known. For various electrolytic oxidation or reduction reactions anyof the usual electrolytes may be utilized within the broader aspect ofthis invention. In the electrolyte reduction of phthalic acid to acyclohexadiene dicarboxylic acid, the invention embraces an anolyte anda catholyte comprising a dilute aqueous acid solution, desirably anaqueous solution of a poly-basic mineral acid, and preferably sulfuricacid in water. A phthalic acid dispersion solution in the catholyte isformed by dissolving phthalic anhydride or phthalic acid in aqueoussulfuric acid. The reduction of phthalic acid to the desired productoccurs by the reaction previously described herein. Since sulfuric acidconcentration builds up in the anolyte and decreases in the anolyte,suitable adjustments are made either intermittently or continuously asdesired.

In order to avoid contamination of the electrolytic cell withextraneously introduced impurities or with side reaction products whichmight poison the mercury electrode, it will be found ad vantageous tocirculate the liquid mercury in the electrode body to and through atreater herein designated as a cell poison separator. The liquid mercuryserves as a carrier which appears to extract poisons as formed inelectrolytic reductions. Thus, the cathode mercury together with mercuryin the conveying system of conduits, vessels and pumps furnishes a meansfor transporting contaminants or cell poisons from the liquid cathodebody to the cell poison separator 39. More particularly the liquidmercury flows from the cathode body l9 through conduit 4| to a levelcontroller 42 for maintaining the fluid level of the liquid electrode ata constant or predetermined height. The mercury carrier then passes fromlevel controller 42 by way of conduit 93 to the mercury treater 39. Thistreater may take one of several forms, such as:

(l) A chemical treater for scrubbing impurities or poisons out of thecarrier or decomposing the same by chemical action;

(2) A still or fractionatinlg column for separating the mercury fromsuch contaminants by distillation; or

(3) A thermal treater for decomposing organic cell poisons by heat whenof the thermally unstable-d type.

The presently preferred form of treater for the circulating mercurycathode comprises a caustic alkali scrubber for removing contaminantsand/or 12 cell poisons in this treater. Aqueous caustic alkali solutionmay be fed to treater 39 by way of line 44 and removed through conduit56. Preferably the chemical treating agent fills at least a substantialportion of a packed treater 39 to provide a relatively deep liquid bodythrough which the mercury carrier is caused to fall in discretedroplets. As the mercury carrier droplets pass through the body ofchemical treating agent contaminants are removed by chemical scrubbingor decomposition. The purified mercury then is collected in the bottomof treater 39 to form a mercury seal which prevents reverse flow andassures discharge of the chemical treating agent upwardly through theseparator countercurrently to the flow of mercury. Mercury from the sealis then passed through outlet conduit 41 to level controller 48 andmercury storage 49 which furnishes a constant supply of mercury forrecirculation to the cathode compartmtnt of the electrolytic cell.Suitable means such as a mercury recirculation pump 5! is provided forreturning the mercury from storage by way of return line 59 to thecathode chamber. As the liquid cathode level is raised by the return ofmercury thereto a portion of the mercury liquid is forced to overflowthrough outlet pipe 4| and again is circulated through caustic scrubber39.

Despite the fact that recycling of electrolyte has been found toaccelerate cell poisoning in processes such as those herein involved,the foregoing system for electrolytic reduction will continue tofunction with recycled electrolyte at an efficient level which may bemaintained for prolonged periods with striking economy in bothelectrolyte and electrolytic current consumption as well as in productconversion and recovery. This recycle type of operation is illustratedin Figure 3 wherein the electrolyte may be returned from product filter38 by way of conduit 52, valve controlled by-pass 53 and storage returnconduit 54 to electrolyte storage tank 55. The stored electrolyte isreturned to the electrolytic cell by way of valve controlled line 55 andmixing heater 33 in the same manner as previously described for freshfeed. Fresh electrolyte and phthalic are introduced into mixer 33 asneeded.

In some instances it will be found advantageous to reduce the color bodycontent of the recycled electrolyte and diminish other adsorba'clecontaminants by treatment with a solid absorbent, such as active carbon,acid treated decolorizing clay, or the like. Provision is made for thismode of operation in Figure 3 wherein active carbon is introduced at'58into mixer 59 and thoroughly contacted with the recycle electrolyte toabsorb impurities contained therein. The resulting electrolyte slurrythen flows through conduit 5| to filter 62 for removal of the activecarbon with its adsorbed impurities and the filtrate is passed tostorage through recycle line 55. Alternatively this treatment with solidadsorbents may be effected before product separation as by insertingmixer 59 and filter 62 in level controller discharge line as where theelectrolyte solution is above crystallization temperature.

In the operation of the apparatus and process of Figure 3 specificprocess conditions will vary among the reactants utilized, and even inthe case of one type of reactant, preferred conditions may change withdifferent cell structures, with ratio of volume of catholyte to surfacearea of cathode, as Well as with other variables such as temperature,electrolyte, and concentration in the catholyte of the chemical compoundto be reduced. However, to illustrate suitable ranges of conditions foroperating the apparatus of Figure 3 in the productionof A3,5-cyclohexadiene transdicarboxylic acid-1,2 the following data aregiven.

Preferred operating temperatures for the catholyte are from about 80 C.to about 90 0., although temperatures as low as about 60 C. and as highas about 100 C. may be utilized. The concentration of sulfuric acid inthe catholyte may vary from about 3% to about 20% by weight ofconcentrated sulfuric acid (specific gravity about 1.84). in water, andthe anolytemay be approximately the same or higher concentration. Fromabout 2% to about 10% preferably approximately 4% by weight of phthalicanhydride is dissolved or dispersedin the aque-- ous sulfuric acidcatholyte solution.

In order to obtain maximum recovery of product it is desirable that 90%or more conversion be effected in the electrolytic cell, and that inproduct recovery the electrolyte should be chilled to just above itsfreezing point to make certain that most of the cyclohexadienedicarboxylic acid is crystallized out and removed by filtration asindicated. Filtrate contains some uncrystallized cyclohexadienedicarboxylic acid or phthalic acid, or both, and is recycled to avoidloss thereof. It has been found that even when the recycled electrolytecontains residual uncrystallized A 3,5-cyclohexadiene transdicarboxylicacid-1,2, this residual acid may be recycled through theelectrolyticcell without substantial further reduction or loss thereof despite itshigh degree of cycloolefinic unsaturation. However, some isomerizationof the A 3,5 acid may occur to. form A 2,6-cyclohexadiene dicarboxylicacid-1,2.

An alternative form of apparatus for carrying out the process of thisinvention is illustrated in Figures 4 and 5 of the drawing. Inspectionof these figures will reveal that the semi-permeable diaphragm of theelectrolytic cell appears in the form of a horizontal cylinder 60 havinga radially protruding lip El at each end thereof to prevent escape ofthe catholyte film. Within the cylindrical diaphragm there is providedan anode 62 immersed in anolyte t3. A cathode chamber is formed by asemi-circular tank 64 containing liquid mercury 66 as the cathode.Catholyte is introduced beneath the surf-ace of the liquid mercurycathode and projected against the cylindrical surface of porousdiaphragm 6.0 by means of inlet conduit 67. The catholyte flows alongthe surface of said diaphragm as indicated and upwardly to the surfaceof the mercury cathode where it is trapped in catholyte compartment 68formed by any suitable means such asaglassplate E9 extending below andabove the mercury surface; Catholyte is removed from the cell throughoverflow conduit H and subjected to further processing ashereindisclosed. If desired, cylinder 5i} may be rotatably mounted tofurnish fresh diaphragm surface in the zone ofu electrolytic reductionand permit washing or any other desired treatment of the diaphragmsurface during the portion of its travel outside the mercury or cathodechamber. Likewise the surface of diaphragm 60 may be fluted or providedwith any suitable form of baffles to insure unidissolved for retainingthe liquid catholyte in contact with form how of the catholyte thereoverand to prevent channelling of the liquid solution.

the porous diaphragm as the catholyte film flows therealong from inletconduit 19 to catholyte outlet Bl. "As in previous descriptions thepreferred cathode 82 is of liquid mercury. The bottom surface ofinclined plate 16 desirably is provided with transverse slots 83 toprevent channelling of catholyte and secure substantially evendistribution of the catholyte film over the inclined porous diaphragmsurface.

In some instances depending upon the compound being treated and thereaction desired, it is possible to substitute other liquid electrodesfor the liquid mercury electrode body. For example, other low meltingmetals or metal alloys may be adopted especially where high hydrogenover-voltage is not essential" to -satisfactory reaction. But where highhydrogen over-voltage is required as in the reduction of phthalic acidto cyclohexadiene"dicarboxylic acid, these metals have not been foundsatisfactory. However, a

."suitableamalgam such as sodium or potassium amalgam is not precludedwhere reduction is being effected in alkaline solution. The alkali metalin the amalgam will react with the electrolyte in this type operation,but additional amalgam is formed by electrolysis during the electrolyticreduction reaction so that it may be said that an amalgam cathodesurface rather than pure mercury is the effective cathode. In such asystem for reducing phthalic acid cyclohexene dicarboxylic acids ratherthan'cyclohexadiene dicarboxylic acids are produced.

In the mercury treating or cell poison separation step alkaline treatingagents other than caustic alkali are operative. Potassium hydroxide isan alternative strong alkaline treating agent for removing impuritiesfrom the mercury carrier. Active oxidizing agents illustratedbypotassium permanganate may be utilized and nitric acid has been foundoperative although it attacks the mercury slightly with resultingincrease in metals consumption.

In the modified form of Figure 8, the catholyte stream 05 enters theelectrolytic cell by way of inlet conduit 36 and is hydrodynamicallybalanced between a semipermeable diaphragm 81 and a solid electrode 88of suitable metal, preferably having a high hydrogen overvoltage, suchas lead. The semipermeable diaphragm 8'! tends to float on theelectrolyte film B5 flowing along its surface and is positioned overelectrode 88 by loosely fitting pins 90. At least the diaphragmelectrolyte and insure proper spreading of the film from center inletconduit 86 outwardly along floating diaphragm 31 to a catholytecollecting "chamber from which the treated solution passes throughconduit 92 to a suitable recovery system. If desired, baffle ribs 89 maybe spiral instead of concentric to provide a spiral channel or conduitalong and through which the electrolyte stream may flow whilesimultaneously contacting the Figures 6 and 7 show a furthermodifiedform- 7aelectrode 88 and semi-permeable diaphragm 87.

The.

- The versatility of the process and apparatus herein described isillustrated by application of the electrolytic cell and process ofFigure 2 to various reactions as hereinafter described.

16 The catholyte eilluent from each of these runs was chilled tocrystallize out the reduced prodnot and the crystals filtered off. Inthe first above run the crystals were contaminated with metallicREDUCTION OF ALDEHYDE GROUP IN 5 mercury accidentally spilled from thecathode GLUCOSE TO GIVE HEXITOLS This reduction was carried outaccording to the following reaction in the electrolytic cell of Figure2:

Glucose to sorbz'tol and manm'tol The conditions chosen for theoperation are set forth in Table. IV.

compartment and the product was therefore not worked up completely. Inthe last two runs above listed the mother liquor was evaporated down toabout 100 cc., chilled, and a second crop of m crystals obtained. Theamounts and inspections of these products are given in Table VI:

Table VI Crop No.

Equiv. F C

During the run the voltage required to main- The melting point of puresuccinic acid is given tain a current of 54 amps. rose from 9.1 to 12.943 in the literature as 170 C. and its equivalent volts and hydrogenevolution which was at first very slight increased as the runprogressed. The hydrogen evolution continued after the current wasinterrupted at the end of the run and weight is 59.0. The constantsgiven in the foregoing table are on unpurified samples but are closeenough to those of chemically pure succinic acid to be conclusive proofof identity of the while the cell was being washed with dilute acid.product when taken with the method of prepara There was, therefore, anappreciable quantity of sodium amalgam formed and dissolved in thecathode. Analysis of the reduced glucose solution revealed a 27%conversion to sorbitol and mannitol. :5 l

REDUCTION OF MALEIC ACID TO SUCCINIC ACID This reduction proceedsaccording to the re action: 55

OHCOOH 2H CHzCOOH CHGOOH CHzCOOH Maleic acid Succinic acid Three runswere made under the conditions (30 given in the following table:

tion.

REDUCTION OF PYRIDINE TO PIPERIDINE This reaction requires a highhydrogen overvoltage cathode and may be regarded as proceeding accordingto the following equation:

GE Cs: 0 CH C CH2 4 11 H A l H CH H: CH1

Pyridine Piperidine Conditions for the run are given in Table VII:

,17. a 18 "Table VII Catholyte Composition Current Feed Rate, Temp:Duration \I, .-1 G Liters/En '7 Percent 35 l ateria rams/llter Amps.Theory "1 V Pyridine 39.5 Q --r---- I r The catholyte product wasrendered alkaline and distillediintil'about 300cc. of aqueousdistillate'had collected. This distillate was diluted to 500bciwitnwater and a cc. aliquot titrated with N/2 sodium hydroxide usinga Becki na'n pH 5 meter. The titration curve revealed a first region ofinfiection' at pH 8and a second inflection-at about pH 4.5. These twoinflections established that the weaker-base (pyridine) was converted inThis pureproduct was identifiedby its melting point asdesoxyca'fieinemonohydr'ate; The mono hydrate-was dried at 200 F. under vacuumandidentified as deso'XycaiT-eine by a melting point determination. Afurther check on the identity of this product was obtaned by convertinga samplej'o f the unrc-crystallized desoxyc'afieine to its pi'crate anda melting point determination part t'oi the stronger base" (piperidine).Distillaelipbn this derivative aJftrr' twice crystaltion bottomscontained di-"and' poly-pipe'ridyls which separateas a viscous brownoil.--

REDUCTION OF CAFFEINE TO DES'QXY CAFFEINE This reaction. was carried.out in accordance with the following equation:

,liz'ing from the water.

' REDUCTION OF NITRO GROUP To AMINO" v GROUP She reduct on of aromaticnitro compounds to amines was carried out by the following exemplary'reaction:

QfialF-C Q, C a C 2 (sac tm' 4H 0: CN OHa NH? on on mo 6H +2H20 OH=N'C'NC'H3N- v Caffeine Desoxycafieine h 1' NEE: I L? Conditions for the runin Table VIII is l1ste I p-Nitroaniline D:Ph nY1 ne'diamine hereinbelow:

4 Table VIII 7 Catholyte Composition Current Feed Rate, Temp, g gLiters/Hr. F. Percent Hm Material Grams/liter Amps. Theory 0aiieine.145. 5 1. 0 90 100 124 0.5 H SO 698 This reaction was effected under thefollowing conditions:

Table IX 0 A Current D t Catholyte omposl' Feed Rate Temp. malon tlon,of Run, Mateml Grams/Liter lhi. Am S Per Cent Hrs.

. Theory p-nitro-aniline c7. 7 1. 0 163 100 127 1.0 H l 90 The reducedcatholyte was diluted with water and neutralized with a slurry ofcalcium hydroxide. The very bulky precipitate of calcium sulfate wasfiltered off, washed with water and The reduced catholyte was chill-edin an ice bath saturated with dry hydrogen chloride and filtered. Theprecipitate was dried under vacuum at 195 F. for twelve hours andidentified as the combination of filtrate washings evaporatedparaphenylene dianiine dihydrochloride.

on a steam bath under vacuum to about 500 cc. The concentrated solutionwas filtered, again extracted several times with chloroform and thechloroform solvent removed by heating in an at- The present invention isnot limited to specific details set forth herein by Way of illustration,but in view of the numerous modifications which may be efiected thereinwithout departmosphere of nitrogen. Th rty-three grams of a ing from thespirit and scope of the invention,

dark colored solid product was obtained. This crude product was purifiedby dissolving in 66 grams of 10% hydrochloric acid and again extractedwith chloroform to remove organic imonly such limitations should beimposed as are recited in the appended claims.

I claim:

1. An electrolytic cell comprising an anode purities. The hydrochloricacid solution was then 70 compartment containing an anode, a cathodemade alkaline with 109 grams of 10% sodium carbonate, treated withactive carbon and extracted with chloroform. Removal of the solvent left16 grams of product which was further puricompartment containing aliquid cathode, a semipermeable diaphragm disposed between said anodecompartment and said cathode compartment, said diaphragm having a facesubject to fled by recrystallization twice from ethyl acetate, the fluidpressure of said liquid cathode, and elec 19 trolyte inlet meansdisposed below said diaphragm for admitting a liquid electrolyte intosaid cathode compartment, said inlet means 156- ing surrounded by saidliquid cathode.

2. An electrolytic cell comprising an anode compartment containing ananode, a cathode compartment containing a liquid mercury cathode, asemi-permeable diaphragm disposed between said anode compartment andsaid cathode compartment, said diaphragm having an irregularface subjectto the fluid pressure of said liquid mercury cathode, an electrolyteinlet means disposed below said diaphragm for admitting a liquidelectrolyte into said cathode compartment, said inlet means beingsurrounded by said liquid mercury cathode.

3. A process for subjecting a liquid electrolyte in the form of acontinuously flowing thin film to the action of an electric currrentwhich comprises forming said film initially by introducing saidelectrolyte into a liquid mercury electrode at a point immediately belowan interface of said electrode and a semi-permeable diaphragm,projecting the electrolyte so introduced against the interfacial surfaceof sa d diaphragm to squeeze said electrolyte into a flowing thin filmbetween said liquid mercury electrode and said diaphragm,

continuing to introduce said electrolyte from the same point to maintainthe flowing film so formed, said film being maintained in contact withthe surface of said diaphragm throughout the process by the fluidpressure of the liquid mercury electrode, and passing an electriccurrent through said continuously flowing thin fllm of electrolytetransversely to the direction of 5 flow thereof.

20 4. The process of claim 3 wherein the liquid electrolyte comprises adispersion of a phthalic "acid in aqueous sulphuric acid solution, saidsolution containing from about 3% to about 20% by weight of sulphuricacid.

PAUL C. CONDI'I'.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 695,302 Gilmour Mar. 11, 1902699,415 Reed May 6, 1902 735,464 Byrnes Aug. 4, 1903 1,209,835Greenawalt Dec. 26, 1916 1,411,507 Paulus Apr. 4, 1922 1,538,389 EwanMay 19, 1925 1,927,289 Kyrides et a1 Sept. 19, 1933 1,981,498 Engelhardtet a1. Nov. 20, 1934 2,307,835 Gardiner Jan. 12, 1943 2,316,685 GardinerApr. 13, 1943 2,336,045 Taylor Dec. 7, 1943 2,477,579 Condit Aug. 2,1949 FOREIGN PATENTS Number Country Date 471,912 Great Britain Sept. 13,1937 OTHER REFERENCES Swann: Transactions of the ElectrochemicalSociety, vol. 69, (1936), page 317.

